The present invention relates to a process for the production of a piezoresistive strain gauge on one lateral face of a flexible beam more particularly belonging to a directional accelerometer, as well as to a process for producing a directional accelerometer equipped with a piezoresistive gauge. These processes use microelectronics technology.
In general terms, an accelerometer essentially comprises a moving mass m and means making it possible to measure the force F=m.A due to the acceleration A of a moving body. A directional accelerometer produced on the basis of microelectronics technology is described in FR-A-2 558 263 in the name of the present Applicant.
FIG. 1 shows in perspective part of the accelerometer described in the aforementioned document. This accelerometer comprises a parallelepipedic substrate 2 having a recess 4 completely traversing the substrate. In said recess are located two parallelepipedic flexible beams 6, 8, whose thickness is much greater than the width (typically 30 times greater). These beams are oriented in a direction Y parallel to the surface of the substrate. These beams have a fixed end integral with substrate 2 and a free end supporting a parallelepipedic block 10.
The displacement of block 10 in direction x is measured with the aid of capacitive detectors, defined by conductive deposits on the lateral faces 12, 14 of block 10 and on the walls of recess 2 facing said faces 12, 14.
This solution has the advantage of being easily brought about according to microelectronics technology. However, these capacitive detectors suffer from a certain number of disadvantages. In particular, they are very sensitive to parasitic capacitances, have a high internal impedance and a non-linear response. However, they have an important influence on the electrostatic forces produced in the accelerometer.
Beams 6 and 8, as well as block 10 are monoblocks and defined by anisotropic etching of substrate 2. They constitute the seismic or moving mass of the accelerometer. Beams 6 and 8 can deform, leading to a displacement of block 10, in a direction x parallel to the surface of the substrate and perpendicular to direction y, said direction x corresponding to the direction of the component of the acceleration to be measured.
The use of piezoresistive strain gauges deposited on the beam of said accelerometer would make it possible to solve the disadvantages associated with capacitive sensors.
It is pointed out that a piezoresistive strain gauge is a conductive strip, whose resistance varies with the deformations of the beam on which it is located. This solution was envisaged in the aforementioned document. The gauges associated in pairs and designated 15 and 17 in FIG. 1 are resistors disposed on the upper face of the beams, which is the only face accessible by conventional micrography processes.
Unfortunately, in this type of accelerometer, the aim is to have a much greater flexibility in direction x (parallel to the surface of the substrate in which the accelerometer is formed and perpendicular to the longitudinal direction of beam y) than in direction y. However, these beams typically have a width of 3 to 10 micrometers for a thickness of a few hundred micrometers. In view of the fact that very little space is available for positioning strain gauges on the upper surface of the beams, serious technical problems are caused by the construction of said gauges.
This is made worse by the fact that strain gauges cannot be positioned along the median longitudinal axis y of beams because on the neutral fibre 16 of the beams, the deformation in direction z perpendicular to the substrate surface is zero. Furthermore, these strain gauges can also not be positioned over the entire length of the beam because, on average, the deformation is zero.
This problem can be solved by utilizing strain gauges of the type shown in FIG. 1 located at both ends of the beams. However, on adding the current supply conductors for these gauges, serious technological problems occur due to the very small dimensions of the elements of the accelerometer. Moreover, assuming that such a structure can be produced, there would still be the problem of heat dissipation, in view of the fact that the gauges only occupy a very small surface on the upper face of the beams.
In view of the problems concerning the available space on the upper face of the beams, the inventors have considered depositing strain gauges on the lateral faces of the beams, oriented parallel to direction y and perpendicular to direction x of detection of an acceleration. Unfortunately, the conventional microelectronics processes using etching masks or deposits parallel to the surface of the substrate do not make it possible to accurately define patterns on the faces perpendicular to the substrate (lateral faces of the beams).